1. Field of the Invention
This invention relates to truss structures and, more particularly, to deployable truss structures desirable for space applications.
2. Description of the Related Art
A truss structure is generally composed of straight members subject only to longitudinal compression, tension or both; the members are disposed so as to make the structure rigid under anticipated loads. Truss members include longerons (longitudinal members); battens (typically short members perpendicular to at least one longeron); and diagonals (bracing members). Typically, the batten members are connected end-to-end to form a batten frame; the batten frames are then joined to each other by the longitudinal members, which are usually attached to the batten frames at the end of the batten members. The diagonals typically are joined between batten frames to opposite longerons.
Truss structures are particularly desirable in space applications because of their high strength and/or stiffness and low mass. The stiffness-to-mass ratio of truss structures may be further increased by the use of composite materials as truss members. The inherently repetitive configuration of truss structures makes efficient packaging possible, allowing the members to be joined by hinges that allow the structure to fold or collapse for compact and efficient storage.
Packaging a truss structure by the use of hinges introduces joint compliance, thereby decreasing the stiffness of the truss. In order to optimize stiffness and strength, the number of hinges should be minimized. Some minimization is achieved by the use of single-degree-of-freedom hinges for complex rotations rather than using ball and socket joints or hinged pairs. Further minimization of hinges may be obtained from designing the truss so that structurally significant truss members (those that affect the bending stiffness) do not hinge except at member junctions (which are referred to as truss nodal positions); these significant truss members are also made relatively long.
Single-degree of-freedom hinges are generally utilized either by having two truss members hinged directly to each other at their ends, or by having two truss members each hinged at their ends to a single common joint structure. The use of single-degree-of-freedom hinges often introduces strain in the truss members during the transition between the collapsed and deployed configurations (which themselves are not strained). The strain during transition arises because a pair of hinges which are at opposite ends of a truss member change in angular orientation with respect to each other. This change in angular orientation is a natural consequence of the design evolution whereby non-folding longerons require batten frame rotation for packaging. It is therefore desirable to reduce or eliminate such strain and joint compliance, while optimizing stiffness in a folding truss structure.